The present invention relates to filled curable polysiloxane encapsulant compositions having bi-modal and tri-modal particle packing distributions of filler particles, in which the filler particles are individually covalently bonded to one or more polysiloxane chains. In particular, the present invention relates to filled curable encapsulant compositions having bi-modal and tri-modal filler particle packing distributions with filler loading levels that maximize the relative bulk volume of the filler particle packing.
In the construction of semiconductor chip package assemblies, it has been found desirable to interpose encapsulating material between or around elements of the semiconductor packages in an effort to reduce or redistribute the strain and stress on the connections between the semiconductor chip and a supporting circuitized substrate during operation of the chip, and to seal the elements against corrosion, as well as to insure intimate contact between the encapsulant, the semiconductor die and the other elements of the chip package.
In the field of elastomeric materials for encapsulating semiconductor chips, most elastomers are poor conductors of heat. This creates difficulties in removing waste heat from semiconductor chips that run hot, such as high-end microprocessors. It also makes it difficult for heat to pass through the package for soldering and de-soldering.
Most elastomers have a high coefficient of thermal expansion (CTE), so that undesirable levels of stress are produced as the various materials used in an semiconductor chip package expand and contract at different rates in response to temperature changes.
Encapsulated metallic conductors such as wire bonds are particularly susceptible to fatigue failures. Typical wire bonds incorporate sharp corners and sharp changes in cross-sectional area at junctions between the fine bonding wire and the connected parts. For example, in a "ball-bond," each fine wire joins a relatively massive ball of wire material at one end. These features tend to create stress concentrations at the junctures. If the wire is flexed repeatedly during service, it can fail at such stress concentrations. When the wire is encapsulated, differential CTE of the chip and encapsulant can cause repeated flexure of the wire and fatigue failure. Attempts have been made to avoid such fatigue failures by using very soft encapsulants such as soft elastomers or gel, at a sacrifice of tensile strength and solvent resistance. Fatigue failure of encapsulated wire bonds remains a significant problem.
A few elastomers have poor solvent resistance. When exposed to certain solvents such as cleaning agents, they swell as they absorb the solvent. Like the stress produced by differences in CTE, the stress produced by solvent swell can also cause package failure. For semiconductor chip packaging, it is important to have good resistance to the cleaning solvents commonly used in the electronics industry, such as esters and terpenes.
U.S. Pat. Nos. 3,649,320; 4,946,878 and 5,001,011 disclose the use of silane coupling agents to improve the mechanical properties of filled thermosetting and thermoplastic resins. A means by which the mechanical properties of curable polysiloxane encapsulant compositions such as Young's modulus and tensile strength may be improved without a sacrifice of thermal conductivity or CTE would be highly desirable.